It's really a sense of scale and how central the recycled bit is. For instance, if you had a whole forestmen fortress, and they were carrying your compound bow, you could in theory replace the compound bow with an old style bow and the MOC would be pretty much the same. If you said "my entry is this cool bow design", otoh, that would not fly. Your Ent, for instance, is an example of something that would be pretty key to a scene. Say you built a MOC of Merry and Pippin meeting Treebeard. If you removed Treebeard, you've just got two short guys out on a hike.

Of course you're the same person you were last month, and you've got pretty much the same bricks, and so chances are you have put some of those bricks together in the same way in the past. Different builders have characteristic styles, and that is in many cases what makes them great. You look at a painting and say "That's a Monet", or "That's a Picasso". So, too, with builders. We're not asking Picasso to start painting fuzzy images of haystacks, or Monet to put both eyes on the same side of the nose. (We can't anyway, they're dead.) But when Monet paints a new picture it is a new picture, even if he did take the same Cathedral as his inspiration, or whatever.